Calcium silicate hydrates ( CSH or C-S-H ) are the main products of the hydration of Portland cement and are primarily responsible for the strength of cement-based materials. They are the main binding phase (the "glue") in most concrete . Only well defined and rare natural crystalline minerals can be abbreviated as CSH while extremely variable and poorly ordered phases without well defined stoichiometry , as it is commonly observed in hardened cement paste (HCP), are denoted C-S-H.
31-400: When water is added to cement, each of the compounds undergoes hydration and contributes to the final state of the concrete. Only calcium silicates contribute to the strength. Tricalcium silicate is responsible for most of the early strength (first 7 days). Dicalcium silicate, which reacts more slowly, only contributes to late strength. Calcium silicate hydrate (also shown as C-S-H) is a result of
62-495: A coagulant in products such as tofu . For the FDA , it is permitted in cheese and related cheese products; cereal flours; bakery products; frozen desserts; artificial sweeteners for jelly & preserves; condiment vegetables; and condiment tomatoes and some candies. It is known in the E number series as E516 , and the UN's FAO knows it as a firming agent, a flour treatment agent,
93-500: A limestone filler addition, CaCO 3 , or CaO·CO 2 , can be noted C C . Hydration products formed in hardened cement pastes (also known as HCPs) are more complicated, because many of these products have nearly the same formula and some are solid solutions with overlapping formulas. Some examples are given below: The hyphens in C-S-H indicate a calcium silicate hydrate phase of variable composition, while 'CSH' would indicate
124-487: A calcium silicate phase, CaH 2 SiO 4 . The cement chemist notation is not restricted to cement applications but is in fact a more general notation of oxide chemistry applicable to other domains than cement chemistry sensu stricto . For instance, in ceramics applications, the kaolinite formula can also be written in terms of oxides, thus the corresponding formula for kaolinite, is or in CCN Although not
155-494: A complex set of calculation known as the Bogue formula . To avoid the flash setting of concrete, due to the very fast hydration of the tricalcium aluminate ( C 3 A ), 2–5 wt. % calcium sulfate is interground with the cement clinker to prepare the cement powder. In cement chemist notation, CaSO 4 ( anhydrite ) is abbreviated as C S , and CaSO 4 ·2H 2 O ( gypsum ) as C S H 2 . Similarly, in case of
186-418: A fire, the structure behind a sheet of drywall will remain relatively cool as water is lost from the gypsum, thus preventing (or substantially retarding) damage to the framing (through combustion of wood members or loss of strength of steel at high temperatures) and consequent structural collapse. But at higher temperatures, calcium sulfate will release oxygen and act as an oxidizing agent . This property
217-413: A given C/S ratio. C-S-H is a nano sized material with some degree of crystallinity as observed by X-ray diffraction techniques. The underlying atomic structure of C-S-H is similar to the naturally occurring mineral tobermorite . It has a layered geometry with calcium silicate sheet structure separated by an interlayer space. The silicates in C-S-H exist as dimers, pentamers and 3n-1 chain units (where n
248-405: A sequestrant, and a leavening agent. Calcium sulfate has a long history of use in dentistry. It has been used in bone regeneration as a graft material and graft binder (or extender) and as a barrier in guided bone tissue regeneration. It is a biocompatible material and is completely resorbed following implantation. It does not evoke a significant host response and creates a calcium-rich milieu in
279-568: A very developed practice in mineralogy, some chemical reactions involving silicate and oxide in the melt or in hydrothermal systems, and silicate weathering processes could also be successfully described by applying the cement chemist notation to silicate mineralogy. An example could be the formal comparison of belite hydration and forsterite serpentinisation dealing both with the hydration of two structurally similar earth -alkaline silicates, Ca 2 SiO 4 and Mg 2 SiO 4 , respectively. The ratio Ca/Si (C/S) and Mg/Si (M/S) decrease from 2 for
310-513: Is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall ), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement. Small amounts of calcined gypsum are added to earth to create strong structures directly from cast earth , an alternative to adobe (which loses its strength when wet). The conditions of dehydration can be changed to adjust
341-466: Is also a common component of fouling deposits in industrial heat exchangers, because its solubility decreases with increasing temperature (see the specific section on the retrograde solubility). The solubility of calcium sulfate decreases as temperature increases. This behaviour ("retrograde solubility") is uncommon: dissolution of most of the salts is endothermic and their solubility increases with temperature.The retrograde solubility of calcium sulfate
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#1732791755741372-587: Is an integer greater than 0) and calcium ions are found to connect these chains making the three dimensional nano structure as observed by dynamic nuclear polarisation surface-enhanced nuclear magnetic resonance . The exact nature of the interlayer remains unknown. One of the greatest difficulties in characterising C-S-H is due to its variable stoichiometry. The scanning electron microscope micrographs of C-S-H does not show any specific crystalline form. They usually manifest as foils or needle/oriented foils. Synthetic C-S-H can be divided in two categories separated at
403-417: Is around 127 million tonnes per annum. In addition to natural sources, calcium sulfate is produced as a by-product in a number of processes: Related sulfur-trapping methods use lime and some produces an impure calcium sulfite , which oxidizes on storage to calcium sulfate. These precipitation processes tend to concentrate radioactive elements in the calcium sulfate product. This issue is particular with
434-429: Is to produce plaster of Paris and stucco . These applications exploit the fact that calcium sulfate which has been powdered and calcined forms a moldable paste upon hydration and hardens as crystalline calcium sulfate dihydrate. It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification. With judicious heating, gypsum converts to
465-405: Is used in aluminothermy . In contrast to most minerals, which when rehydrated simply form liquid or semi-liquid pastes, or remain powdery, calcined gypsum has an unusual property: when mixed with water at normal (ambient) temperatures, it quickly reverts chemically to the preferred dihydrate form, while physically "setting" to form a rigid and relatively strong gypsum crystal lattice: This reaction
496-430: Is why "-" is used between C, S, and H. Synthetic C-S-H can be prepared from the reaction of CaO and SiO 2 in water or through the double precipitation method using various salts. These methods provide the flexibility of producing C-S-H at specific C/S (Ca/Si, or CaO/SiO 2 ) ratios. The C-S-H from cement phases can also be treated with an ammonium nitrate solution in order to induce calcium leaching, and so to achieve
527-493: The formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium , silicon , and various metals . The main oxides present in cement (or in glass and ceramics) are abbreviated in the following way: For the sake of mass balance calculations, hydroxides present in hydrated phases found in hardened cement paste, such as in portlandite , Ca(OH) 2 , must first be converted into oxide and water. To better understand
558-454: The Ca/Si ratio of about 1.1. There are several indications that the chemical, physical and mechanical characteristics of C-S-H varies noticeably between these two categories. Other C-S-H minerals: Other calcium aluminium silicate hydrate, (C-A-S-H) minerals: Mechanisms of formation of C-S-H phases: Cement chemist notation Cement chemist notation ( CCN ) was developed to simplify
589-440: The area of implantation. When sold at the anhydrous state as a desiccant with a color-indicating agent under the name Drierite , it appears blue (anhydrous) or pink (hydrated) due to impregnation with cobalt(II) chloride , which functions as a moisture indicator. Up to the 1970s, commercial quantities of sulfuric acid were produced from anhydrous calcium sulfate. Upon being mixed with shale or marl , and roasted at 1400°C,
620-495: The completely anhydrous form called β-anhydrite or "natural" anhydrite is formed. Natural anhydrite does not react with water, even over geological timescales, unless very finely ground. The variable composition of the hemihydrate and γ-anhydrite, and their easy inter-conversion, is due to their nearly identical crystal structures containing "channels" that can accommodate variable amounts of water, or other small molecules such as methanol . The calcium sulfate hydrates are used as
651-414: The composition of: Four main phases are present in the clinker and in the non-hydrated Portland cement . They are formed at high temperature (1,450 °C) in the cement kiln and are the following: The four compounds referred as C 3 S, C 2 S, C 3 A and C 4 AF are known as the main crystalline phases of Portland cement. The phase composition of a particular cement can be quantified through
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#1732791755741682-716: The concise formalism of the cement chemist notation in their works. Calcium sulfate Calcium sulfate (or calcium sulphate ) is the inorganic compound with the formula CaSO 4 and related hydrates . In the form of γ- anhydrite (the anhydrous form), it is used as a desiccant . One particular hydrate is better known as plaster of Paris , and another occurs naturally as the mineral gypsum . It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water. The compound exists in three levels of hydration corresponding to different crystallographic structures and to minerals: The main use of calcium sulfate
713-493: The conversion process of hydroxide anions in oxide and water, it is necessary to consider the autoprotolysis of the hydroxyl anions; it implies a proton exchange between two OH , like in a classical acid–base reaction : or also, For portlandite this gives thus the following mass balance: Thus portlandite can be written as CaO · H 2 O or CH. These oxides are used to build more complex compounds . The main crystalline phases described hereafter are related respectively to
744-535: The dicalcium and dimagnesium silicate reagents to 1.5 for the hydrated silicate products of the hydration reaction. In other term, the C-S-H or the serpentine are less rich in Ca and Mg respectively. This is why the reaction leads to the elimination of the excess of portlandite (Ca(OH) 2 ) and brucite (Mg(OH) 2 ), respectively, out of the silicate system, giving rise to the crystallization of both hydroxides as separate phases. The rapid reaction of belite hydration in
775-431: The gypsum at this time (the heat of hydration) tends to go into driving off water (as water vapor) rather than increasing the temperature of the mineral, which rises slowly until the water is gone, then increases more rapidly. The equation for the partial dehydration is: The endothermic property of this reaction is relevant to the performance of drywall , conferring fire resistance to residential and other structures. In
806-508: The partially dehydrated mineral called bassanite or plaster of Paris . This material has the formula CaSO 4 ·( n H 2 O), where 0.5 ≤ n ≤ 0.8. Temperatures between 100 and 150 °C (212–302 °F) are required to drive off the water within its structure. The details of the temperature and time depend on ambient humidity. Temperatures as high as 170 °C (338 °F) are used in industrial calcination, but at these temperatures γ-anhydrite begins to form. The heat energy delivered to
837-428: The phosphate by-product, since phosphate ores naturally contain uranium and its decay products such as radium-226 , lead-210 and polonium-210 . Extraction of uranium from phosphorus ores can be economical on its own depending on prices on the uranium market or the separation of uranium can be mandated by environmental legislation and its sale is used to recover part of the cost of the process. Calcium sulfate
868-431: The porosity of the hemihydrate, resulting in the so-called α- and β-hemihydrates (which are more or less chemically identical). On heating to 180 °C (356 °F), the nearly water-free form, called γ-anhydrite (CaSO 4 · n H 2 O where n = 0 to 0.05) is produced. γ-Anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial desiccants . On heating above 250 °C,
899-413: The reaction between the silicate phases of Portland cement and water. This reaction typically is expressed as: also written in cement chemist notation , (CCN) as: or, tricalcium silicate + water → calcium silicate hydrate + calcium hydroxide + heat The stoichiometry of C-S-H in cement paste is variable and the state of chemically and physically bound water in its structure is not transparent, which
930-475: The setting of cement is formally "chemically analogue" to the slow natural hydration of forsterite (the magnesium end-member of olivine ) leading to the formation of serpentine and brucite in nature. However, the kinetic of hydration of poorly crystallized artificial belite is much swifter than the slow conversion/weathering of well crystallized Mg- olivine under natural conditions. This comparison suggests that mineralogists could probably also benefit from
961-466: The sulfate liberates sulfur dioxide gas, a precursor to sulfuric acid . The reaction also produces calcium silicate , used in cement clinker production. Some component reactions pertaining to calcium sulfate: The main sources of calcium sulfate are naturally occurring gypsum and anhydrite , which occur at many locations worldwide as evaporites . These may be extracted by open-cast quarrying or by deep mining. World production of natural gypsum